In electrostatography, an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element and is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface following development. The toner image may be transferred to a receiver, to which it is fused, typically by heat and pressure.
Toners contain a binder and other optional additives, such as colorants and charge control agents. Electrostatographic toners are commonly made by polymerization of a binder followed by mixing the binder polymer with various additives and then grinding to a desired size range.
The toner binder has a number of characteristics which are influenced by its molecular weight and its architecture. For example, the molecular weight distribution and degree of branching or crosslinking directly affect toner properties such as melt viscosity, melt elasticity, toner keeping, photoconductor scumming, fusing and grinding characteristics. Therefore, it is desirable to control the molecular weight and molecular architecture of electrophotographic binder polymers quite precisely.
One of the most effective ways to achieve control over molecular weight and chain architecture is through the well-known use of mercaptan-type (RSH) chain-transfer agents which are added directly to the monomer mixture prior to initiation of the polymerization reaction. These types of chain-transfer agents possess high transfer coefficients (Cs) for styrene-based systems. They provide excellent control over polymer molecular weight and branching when they are used in conjunction with a crosslinking agent such as divinylbenzene.
The use of RSH-type chain-transfer agents suffers from a significant drawback. The mercaptan groups are incorporated into the polymer chains mainly as sulfide. In toner manufacture the binder polymers are subjected to high temperatures and shear resulting in degradation of the long polymer chains. This phenomenon leads to the regeneration of free thiol compounds from the chain ends as well as other thiol-containing species. Some of these thermally-generated species are low enough in molecular weight to possess appreciable vapor pressures both at ambient temperatures and particularly when present in high-heat areas of electrophotographic copiers such as fusing stations.
Since these thiols and/or sulfide compounds have strong, objectionable odors typical of most sulfur compounds, their presence as impurity in the toner is not desirable. These sulfur compounds can have odor thresholds as low as 2 parts per billion. Additionally, these thermally-generated thiol compounds can further be oxidatively degraded to yield sulfur dioxide (SO.sub.2) which is a highly volatile, malodorous gas.
Further, the preparation of commercial toner binder polymers often involves the addition polymerization of volatile vinylic compounds in the presence of suitable free radical initiators and molecular weight distribution modifiers as further discussed below. Suitable vinylic compounds for toner binder synthesis include, but are not limited to, a combination of styrene, vinyl toluene, t-butyls-tyrene, butyl acrylate, butyl methacrylate, iso-butyl methacrylate, 2-hydroxyethyl methacrylate, acrylic acid, methacrylic acid and may furthermore be crosslinked with suitable tetrafunctional vinylic moieties such as divinyl benzene to further augment desirable toner binder properties.
If these monomers are polymerized into the high molecular weight polymer chain then their vapor pressure essentially drops to zero. However, if as is most often the case, conversion of vinyl monomer to polymer is incomplete then the resulting binder polymer will suffer from a substantial odor problem caused by the presence of all the volatiles resulting from unpolymerized vinyl, acrylate and methacrylate monomers.
The styrenic moieties styrene, vinyl toluene, t-butylstyrene have particularly objectionable odors with very low thresholds of olfactory detection. These odors can be substantially aggravated in the heated zones of copiers such as in fusing stations. Odor thresholds for these compounds are so low that conventional methods of polymer purification such as solvent washing, reprecipitation, slurrying etc., cannot provide odor-free toner binder.
The toner composition generally also comprises optional addenda with various functions. For example, charge control agents are typically used in toner compositions to provide the desired charge level as well as to provide charge stability over a long developer life. Various pigments and dyestuffs are also used to provide the preferred hue and hiding power to the marking toner. Further, various low molecular weight polyolefins and aliphatic amines or aliphatic acids or salts thereof are used very commonly to provide release characteristics to the toner from the fuser roller surface etc. All such additives and other optional additives not mentioned above have their characteristic odor which are often quite objectionable when toners comprising them are fused by the application of heat. Often, the odor is easily detectable when a stored toner bottle is opened prior to placing it in the copier.
What is needed, then, is a product and/or process of eliminating the objectionable odors associated with vinyl addition polymerization especially in the presence of mercaptan-mediated chain-transfer agents and other optional toner additives.